Method of unplugging fuel-cell oxygen inlets and the like



United States Patent ice 7 3,471,333 METHOD OFUNPLUGGING FUEL-CELLOXYGEN INLETS'AND THE LIKE Robert Lee Novack, Hanover, Mass., assignorto Prototech Company, division of Bolt Beranek and Newman Inc.,Cambridge, Mass., a corporation of Massachusetts No Drawing. Filed Aug.9, 1967, Ser. No. 659,295 Int. Cl. H01m 27/00 US. Cl. 136 86 4 ClaimsABSTRACT OF THE DISCLOSURE A novel method is disclosed of unpluggingoxide deposits in the region where electrochemical cell oxygen inletscontact the electrolytic medium involving the periodic introduction of acritical quantity of steam.

The present invention relates to methods of unplugging oxygen inlets inelectrochemical cells and the like, being more particularly, though notexclusively, directed to methods of unplugging oxide deposits developedat the region where oxygen inlets to fuel cells and the like contact anelevated temperature substantially anhydrous electrolytic medium.

Particularly in anhydrous alkaline-hydroxide fuel cells and the like,wherein an oxidant such as oxygen or air (hereinafter genericallyreferred to as oxygen) is introduced in the cathode region of theelectrolytic medium, prolonged operation has been found to result in thelodging of oxide deposits apparently from the electrolytic medium oradditives associated therewith at the region where the oxygen inletcontacts the electrolytic medium. While the cell may be shut down andthe oxygen inlet unplugged, this is not a satisfactory solution forapplications where continuous operation over long periods of time isrequired.

An object of the invention, accordingly, is to provide a new andimproved method of unplugging such oxide deposits that is adapted to beused without substantially disrupting the continuous operation of thecell, without necessitating the shut down or disassembly of the cell,and without poisoning or otherwise destroying or modifying the efficacyof the electrolytic medium employed in the cell. In summary, this end isachieved through the periodic introduction of steam, preferably from apoint external to the cell, of quantities suflicient to loosen the oxidedeposits, but insufficient to change the water content or otherproperties of the electrolytic medium.

Other and further objects will be explained hereinafter and will be moreparticularly delineated in the appended claims.

Preferred cells subject to the above-described difliculties include400-600 C. substantially anhydrous molten alkaline-hydroxideelectrolytic media provided with an anode, such as apalladium-containing hydrogenpermeable, but otherwise non-porous layer,and a preferably non-porous nickel cathode structure which may, indeed,comprise the cell housing, itself. Oxygen is fed into the medium in thecathode region to agitate the same and to insure the continualgeneration of peroxides and superoxides within the medium that serve asthe cell oxidant, and with a barrier preferably interposed between theanode and cathode that is pervious to the ions of the electrolyte, butof suflicient dimensions and of sufficiently restricted porosity toprevent substantial agitation transfer from the oxygen-agitated cathoderegion to the anode region, as described, for example, in co-pendingapplication of David Moulton, Robert Novack and Walter Juda, Ser. No.539,768, filed Apr. 4, 1966,

3,471,333 Patented Oct. 7, 1969 for Fuel Cell Process and Apparatus. Asdescribed therein, cells of the above type are operated with theelectrolytic medium in contact with an oxygen-containing atmosphere andat a temperature of at least substantially 300 degrees C. and suflicientto render the medium molten and substantially anhydrous, with theperoxide and superoxide functioning as substantially the sole fuel cellelectrochemical oxidant to provide substantially the entire currentoutput of the fuel cell.

In such cells, a convenient way of introducing the oxygen is by means oftubing, as, for example, of nickel, that bubbles oxygen into the regionof the cathode structure. After several days of operation, however, itwill customarily be noted that the inlet flow rate of the oxygen becomesdecreased and, indeed, the power density of the cell drops off. This hasbeen: traced to oxide deposits that have been found to develop in suchperiods of time in the region of the inlet tube contacting theelectrolytic medium, plugging the inlet. These deposits are extremelyhard and difficult to remove, particularly during the continuedhigh-temperature operation of the cell. Attempts, indeed, to snake awire into the inlet tube to try to dislodge the plugging oxide depositshave proven ineffectual.

In accordance with the present invention, it has been found that thesedeposits may be rapidly loosened and bubbled away by the periodicintroduction of steam at the plugged ragion. If, however, too much watervapor is introduced into the electrolytic medium, this will tend todestroy the substantially anhydrous state of the medium required foroptimum peroxide and superoxide information. It has been found that theintroduction of steam must be controlled to provide a quantity onlysuflicient to loosen the oxide, but insuflicient to cause theelectrolyte medium at the inlet region to change its substantiallyanhydrous state.

As an example, in a cell operating with about 500 grams of percent KOHand 10 percent NaOH as the electrolytic medium, maintained at atemperature of 475 C., and having hydrogen fuel applied through aplatinumcontaining anode layer and oxygen introduced in the form of airbubbled through a %-inch nickel tube at the cathode region at a rate of500 ml./min., the air inlet flow rate and the power density willdeteriorate as the result of such plugging within about four days ofoperation. If, in response to such degradation, at the end of each suchperiod of time, one-to-two drops of water (about 0.05-0.1 ml.) areexternally inserted into the oxygen inlet, it has been found that thewater droplets become converted into steam in dropping through the inlettube as the result of the elevated temperature of the cell, and fullyloosen the oxide to unplug the inlet, but with insufficient residualwater vapor resulting to alter the desired substantially anhydrous stateof the electrolytic medium.

As another example, limited water vapor has been introduced into the airat the inlet, the steam being at a partial pressure of about 400 mm. ofmercury (total pressure about 1 atmosphere), obtaining the satisfactoryunplugging without substantially altering the anhydrous state of theelectrolytic medium (about 500 grams) over about two minutes at anair-steam flow of 500 ml./minute.

Clearly, any well-known, water-mixing or droplet-introducing apparatus,such as mixers, syringes, or the like, may be employed and adaptedeither to operate perodically at fixed intervals of time or periodicallyin response to degradation of low rate or power density, by thoroughlywell-known feed-back or sensing controls and techniques. Further,modifications will also occur to those skilled in this art.

What is claimed is:

1. A method of operating a fuel cell containing an alkali-metalhydroxide electrolyte medium with an oxygen inlet in contact therewith,that comprises maintaining the medium at a temperature in excess ofabout 300 C. and suflicient to render the medium molten andsubstantially anhydrous and in contact with an oxygen-containingatmosphere in an amount sufficient to provide in the medium at least oneof alkali-metal peroxide and superoxide, said peroxide and superoxidefunctioning as substantially the sole fuel cell electrochemical oxidant,applying hydrogen-containing fuel to the fuel cell, drawing current fromsaid fuel cell while holding the said medium at such temperature andwhile utilizing said oxidant electrochemically to provide substantiallythe entire current output of the fuel cell, whereby oxide deposits tendto plug said inlet region, and periodically introducing steam to theregion of said oxygen inlet in contact with said medium in sufficientquantity to loosen the oxide at said inlet region but insufficient tocause the medium at said region to change its substantially anhydrousstate.

2. A method as claimed in claim 1 and in which said steam is produced bywater droplets introduced into the said inlet and vaporized in passingthrough the same into the elevated-temperature medium.

3. A method as claimed in claim 1 and in which said steam-introducingstep is effected in response to degradation in at least one of inletoxygen flow rate and power density.

4. A method as claimed in claim 1 and in which the elevated temperatureis in excess of about 400 C.

References Cited UNITED STATES PATENTS 2,060,663 11/1936 De Walt.

3,330,699 7/1967 Tantram 13686 3,350,226 10/1967 Lieb et a1. 136-863,403,056 9/1968 Barde et al. 13686 WINSTON A. DOUGLAS, Primary ExaminerM. J. ANDREWS, Assistant Examiner

